Light emitting devices, which may be generally classified as organic or inorganic, are well known in the graphic display and imaging art. Among the benefits of organic light emitting devices are high visibility due to self-emission, as well as superior impact resistance, and ease of handling of the solid state devices. Organic light emitting display devices may have practical application for television and graphic displays, as well as in digital printing applications.
An organic light emitting display device is typically a laminate formed on a substrate such as soda-lime glass. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The semiconductor layers may be hole-injecting and electron-injecting layers. The light-emitting layer may be selected from any of a multitude of light emitting organic solids. The light-emitting layer may consist of multiple sublayers.
When a potential difference is applied across the cathode and anode, electrons from the electron-injecting layer, and holes from the hole-injecting layer are injected into the light-emitting layer. They recombine, emitting light.
In a typical matrix-addressed organic light emitting display device, numerous light emitting devices are formed on a single substrate and arranged in groups in a regular grid pattern. Several light emitting device groups forming a column of the grid may share a common cathode, or cathode line. Several light emitting device groups forming a row of the grid may share a common anode, or anode line. The individual light emitting devices in a given group emit light when their cathode and anode are activated at the same time. Activation may be by rows and columns or in an active matrix with individual cathode and anode pads.
Organic light emitting devices have a number of beneficial characteristics. These include a low activation voltage (about to 4.5 volts), fast response when formed with a thin light-emitting layer, and high brightness in proportion to the injected electric current. By changing the kinds of organic solids making up the light-emitting layer, many different colors of light may be emitted, ranging from visible blue, to green, yellow, and red. Organic light emitting devices are currently the subject of aggressive investigative efforts.
Organic light emitting devices need to be protected from the atmosphere. The light emitting organic material in the light-emitting layer can be highly reactive. The material is susceptible to water, oxygen, etc. Moisture and oxygen may cause a reduction in the useful life of the light emitting device. The cathode and anode may also be affected by oxidation.
One disadvantage of oxygen and moisture penetration into the interior of the organic light emitting device is the potential to form metal oxides at the metal-organic interface. These metal oxide impurities may allow separation of the cathode or anode and the organic in a matrix addressed OLED, especially the oxidation sensitive cathode, such as, Mg+Ag or Al+Li. This can result in the formation of dark non-emitting spots (i.e., no illumination) because no current flows through the area of the separation.
Edge shorting between the cathode and anode layers is a current problem affecting most conventional organic light emitting display devices. This edge shorting reduces the illuminating potential of the display devices.
Exposing a conventional light emitting device to the atmosphere, shortens its life. To obtain a practical, useable organic light emitting device, it is necessary to protect the device, so that water, oxygen, etc., do not infiltrate the light-emitting layer.
Methods commonly employed for protecting or sealing inorganic light emitting devices are typically not effective for sealing organic light emitting devices. Resin coatings that have been used to protect inorganic light emitting devices are not suited for organic light emitting devices. The solvent used in the resin coating solution tends to infiltrate the light-emitting layer, degrading the light emission properties of the device. It alters the organic light-emitting layer, reducing or eliminating its light emission properties.